1. Field of the Invention
This invention relates to three dimensional woven fabrics comprising pitch-derived carbon fibers having superior qualities. More particularly, it relates to three dimensional woven fabrics comprising easily graphitizable pitch-derived carbon fibers which are incomplete in crystallization and orientation of graphite crystals and yet have the capability of increasing their tensile strength and modulus of elasticity greatly by heat treatment carried out under a relaxed state whereby crystal growth and orientation proceed. "Three dimensional", herein referred to, means that the directions of three component yarns, such as, warp, weft and vertical yarns, are essentially perpendicular to each other.
This invention is directed to three dimensional woven fabrics comprising high strength, high elasticity carbon fibers derived from pitch which fibers are difficult to weave after heat treatment.
This invention is directed to three-dimensional fabrics woven from components at least one of which is easy for working and is a pitch-derived carbon fiber lightly carbonized at a lower temperature than generally used temperature. In case of light carbonization fibers, three dimensional weaving containing bent parts of a small radius of curvature can be carried out, which is difficult in case of fibers having a higher degree of carbonization (heat treatment). In addition, since the light carbonization fibers themselves are lower in cost, even when working loss is formed, the relative influence upon product cost is small. This is also an advantage of the light carbonization fibers.
The light carbonization fibers of the present invention are more easily bent at parts of small radius of curvature compared with carbon fibers having higher grade of carbonization, and have superior characteristic properties because their bent parts receive stress relaxation by the higher grade of carbonization treatment applied thereafter and show superior resistances to abrasion, flexion and scratching. Therefore, the three dimensional woven fabrics of the present invention show superior ability as reinforcement fabrics for various kinds of materials.
2. Description of prior Arts
This invention is directed to three dimensional woven fabrics of pitch-derived carbon fibers having high modulus of elasticity, useful for fiber reinforcement.
Pitch-derived carbon fibers having high modulus of elasticity have been heretofore produced firstly by subjecting a pitch having a high softening point to melt-spinning, oxidizing the surfaces of resulting fibers to make them infusible (thermoset), followed by carbonization carried out in an inert gas atmosphere.
This method is disclosed in Japanese official gazette of examined applications (Tokuko) 15728 of 1966. This is certainly a superior production method of pitch-derived fibers, but according to the disclosed method, it is necessary to keep fibers under tension at the time of carbonization to obtain fibers having high modulus of elasticity. Since thermoset pitch fibers are extremely brittle, it is difficult to hold them under tension. It is considered actually to be impossible to obtain high elasticity fibers by this method.
In order to work out a solution to this problem, a method in which optically anisotropic pitch is used has been proposed as disclosed in Japanese official gazette of examined applications (Tokuko) 8634 of 1974 and Japanese official gazette of unexamined applications (Tokukai) 19127 of 1974. Optically anisotropic pitch is an easily graphitizable material and shows superior properties as raw material for high strength, high elasticity carbon fibers. Particularly, there is no need of being kept under tension at the time of carbonization, they are considered to be advantageous method in terms of cost and quality.
Carbon fibers from optically anisotropic pitch can be easily made into a high strength, high elasticity state, but on the other hand they have such a weak point that they are liable to be flawed, e.g. liable to be broken at the time of working. Such weak points exist more or less in case of all brittle fibers. Glass fibers, PAN-derived carbon fibers, etc. are coated by sizing agents to give lubricity and cohesiveness of bundles. In case of carbon fibers from optically anisotropic pitch, there is a tendency to repel a sizing agent due to harmful effect of easily graphitizable property. since uniform coating is difficult, shortages of lubricity and cohesiveness of bundles are also weak points.
In order to solve these problems, Japanese unexamined patent application (Tokukai) 21911 of 1985 discloses a method in which primary carbonization should be carried out at a temperature of 400.degree.-650.degree. C. after thermosetting. This method is effective to some-extent for keeping elasticity of carbon fibers small and for preventing them from being flawed, but since carbonization degree is too light, there are problems in insufficiency of shape and dimensional stability and in sufficiency of strength for passing through a process such as weaving in which strong force is to be applied.
Particularly in case of three dimensional woven fabrics, since there is a stage in which bending is applied at a small radius of curvature at the time of weaving, there is a problem of difficulty of weaving for high strength, high elasticity carbon fibers which are highly carbonized or graphitized. In case of high elasticity fibers having an elongation less than 0.4% and a modulus of elasticity higher than 40,000 Kgf/mm.sup.2, simple fabrics such as plain woven fabrics can be woven, but it has been found to be substantially impossible to weave three dimensional woven fabrics containing bent parts having a radius of curvature less than about 4 mm using a weaving machine.
The present invention relates to three dimensional woven fabrics comprising carbon fibers having high elasticity, produced from an optically anisotropic pitch or a high softening point pitch having a carbonization characteristic property close to an optically anisotropic pitch. It has been difficult heretofore to produce such three dimensional woven fabrics comprising carbon fibers having a modulus of elasticity higher than 40,000 Kgf/mm.sup.2, because of brittleness of such high elasticity carbon fibers and shortage of lubricity and cohesiveness of bundles, etc. PAN-derived carbon fibers generally have a higher strength but lower elasticity than pitch-derived carbon fibers and the elasticity of PAN-derived carbon fibers cannot be increased by a heat treatment under a relaxed state. Therefore, it may be possible to weave three dimensional fabrics with PAN-derived fibers having a modulus of elasticity of up to 40,000 Kgf/mm.sup.2, it is impossible to increase a modulus f elasticity to a value higher than 40,000 Kgf/mm.sup.2 by an additional heat treatment under relaxed state, because woven fabrics cannot be stretched during the heat treatment.
It is an object of the present invention to solve the above-mentioned points of problem.
The present invention resides in three dimensional fabrics woven by using carbon fibers having good workability and low strength and low elasticity, obtained by mitigating the carbonization condition of pitch fibers. The carbonization of pitch fibers are generally carried out by heat-treatment under an inert gas atmosphere. In case where high modulus of elasticity is required, heat treatment is carried out at a temperature higher than 2000.degree. C. It has been found, however, that workability of such high strength, high elasticity carbon fibers is not good and carbonization carried out at a lower temperature is preferable.